1. Field of the Invention
The present invention relates to the field of television signal encoding. In particular, the invention relates to an encoding algorithm, encoder and decoder enabling transmission of broadcast quality television signals in the form of an embedded code.
2. Related Art
In the television studio there is a continuing trend towards the replacement of analogue, composite picture handling by digital manipulation of separate luminance and chrominance components. This is performed at the internationally agreed rate of 166M bit/s, with a further 50M bit/s representing the redundant picture blanking. A full television "package" however must also comprise audio channels, teletext, synchronisation and control signals which further elevate the source rate. Independently digitisation of the European telecommunications networks is taking place based upon a binary hierarchy which stretches from the trunk rate of 140M bit/s to the rate of a single telephone conversation. So if the broadcaster wishes to convey pictures between studios, to radio transmitters or perhaps directly to the home using these links he is faced with the task of converting his source rate to conform to one of the levels in the telecoms multiplex hierarchy. In particular he will wish to use the most economical level which allows him to achieve adequate quality reproduction.
The broadcaster can however identify two distinct levels of reproduction fidelity required from a coding algorithm, these are "contribution" quality for interstudio traffic and "distribution" quality for traffic destined for the home. Each suggests a coding algorithm optimised for different criteria and each a quite different transmission rate. Contribution material should be of studio resolution and of such numerical fidelity that it can withstand post processing, for example by standards conversion, chromakey or multiple passes through the same coding algorithm. We might aim for a transmission rate of 68M bit/s for this. Distribution material (and perhaps news contribution from remote locations) need not be post-processed, but instead conveyed directly to the viewer. Here we can be sure that successive codes will not be encountered and so we are free to exploit the psychovisual redundancy present in the images and aim for a channel rate of 34M bit/s. This rate is emerging as a popular choice for the proposed exchange of pictures within Europe.